The present invention relates to a process for the preparation of homopolymers and copolymers of alpha monoolefins at from 20.degree. to 160.degree. C., especially from 50.degree. to 120.degree. C., and under a pressure of from 1 to 100, especially from 20 to 70, bar, by means of a catalyst system comprising
(1) a titanium halide of the formula EQU TiCl.sub.3.m AlCl.sub.3
where m is a number from 0 to 0.5, especially from 0.1 to 0.4, and
(2) an aluminum-alkyl of the formula ##STR1## where X and Y are each alkyl of not more than 8, especially not more than 2, carbon atoms, and Z is chlorine or alkyl of not more than 8, especially not more than 2, carbon atoms.
Specific modifications of the catalyst system may be made in order to achieve particular objectives, for example the following:
(a) Catalyst systems which can be incorporated into the polymerization reactor in situ and do not require expensive and time-consuming grinding processes as disclosed in U.S. Pat. Nos. 4,120,823, 4,154,699, and 4,154,700, issued Oct. 17, 1978 and May 15, 1979, respectively.
(b) Catalyst systems which on polymerization of alpha monoolefins, especially propylene, give polymers with a relatively high proportion of stereoregular (=isotactic) polymer.
(c) Catalyst systems which can give an increased yield of polymer, namely systems of increased productivity (systems where the amount of polymer formed per unit weight of catalyst system is increased).
(d) Catalyst systems which introduce less halogen into the polymer, which is achievable by increasing the yield according to (c) and/or by employing a titanium halide which contains very little halogen.
(e) Catalyst systems which retain a constant or relatively constant activity maximum over a very long time, which is of substantial importance for the catalyst yield.
(f) Catalyst systems which make it possible, by increasing the polymerization temperature, to increase the conversion without a significant reduction in the stereoregularity of the polymers, an effect which is generally desirable, especially in dry phase polymerization.
(g) Catalyst systems by means of which--especially at relatively high polymerization temperatures--the morphological properties of the polymers can be influenced in a particular way, for example, in giving a uniform particle size and/or reducing the proportion of fines and/or giving a high bulk density. These factors may, for example, be significant in respect of technical control of the polymerization system, of working of the polymers, and/or processability of the polymers.
(h) Catalyst systems which are simple and safe to prepare and easy to handle; for example, systems which can be prepared in (inert) hydrocarbon auxiliary media.
(i) Catalyst systems which make it possible, where the polymerization is carried out in the presence of a molecular weight regulator, especially hydrogen, to manage with relatively small amounts of regulator. This can be significant, for example, in respect to the thermodynamics of the process.
(j) Catalyst systems which are tailored for specific polymerization processes, for example, catalysts which are suited either to the specific peculiarities of suspension polymerization or to the specific peculiarities of dry phase polymerization.
(k) Catalyst systems which give polymers having a pattern of properties which makes them particularly suitable for one or another field of use.
Experience to date has shown that amongst the various objectives there are some which can only be achieved by special embodiments of the catalyst system if other objectives are lowered. Under these circumstances it is, in general, desirable to find embodiments which not only achieve the particular objectives but also demand minimum lowering of other desirable objectives.
It is an object of the present invention to provide a novel embodiment of a catalyst system by means of which better results can be achieved--for similar objectives--than with conventional embodiments. We have found that this object is achieved with a catalyst system of the type defined at the outset, which contains, as a further component (3), a Lewis base having at least two pairs of electrons to donate, particularly esters of carboxylic acids having oxygen, nitrogen, or sulfur-containing heterocyclic structures.
U.S. Pat. No. 4,255,280 issued Mar. 10, 1981, discloses catalysts for polymerization of olefins which comprise a solid catalyst component, which is a support material, an organometallic compound, and a heterocyclic carboxylic acid ester which contains nitrogen, oxygen, or sulfur. The catalysts disclosed in that patent require an organomagnesium compound and a catalyst support material, neither of which are present in nor required for the catalyst of the present invention. My catalyst can be prepared directly from commercially available titanium trichloride products. French Pat. No. 2,253,029 discloses that the addition of 2-pyrrolecarboxaldehyde or 1-methyl-2-pyrrolecarboxyaldehyde to standard polyolefin catalysts increases the isotacticity of the stereospecific polymers produced. These compounds both have more than one pair of electrons to donate, but neither of them is a carboxylic acid ester.